Abstract
In depositional intertidal coastal systems, primary production is dominated by benthic microalgae (microphytobenthos) inhabiting the mudflats. This benthic productivity is supporting secondary production and supplying important services to humans including food provisioning. Increased frequencies of extreme events in weather (such as heatwaves, storm surges and cloudbursts) are expected to strongly impact the spatiotemporal dynamics of the microphytobenthos and subsequently their contribution to coastal food webs. Within north-western Europe, the years 2018 and 2019 were characterized by record-breaking summer temperatures and accompanying droughts. Field-calibrated satellite data (Sentinel 2) were used to quantify the seasonal dynamics of microphytobenthos biomass and production at an unprecedented spatial and temporal resolution during these years. We demonstrate that the Normalized Difference Vegetation Index (NDVI) should be used with caution in depositional coastal intertidal systems, because it may reflect import of remains of allochthonous pelagic productivity rather than local benthic biomass. We show that the reduction in summer biomass of the benthic microalgae cannot be explained by grazing but was most probably due to the high temperatures. The fivefold increase in salinity from January to September 2018, resulting from reduced river run-off during this exceptionally dry year, cannot have been without consequences for the vitality of the microphytobenthos community and its resistance to wind stress and cloud bursts. Comparison to historical information revealed that primary productivity of microphytobenthos may vary at least fivefold due to variations in environmental conditions. Therefore, ongoing changes in environmental conditions and especially extreme events because of climate change will not only lead to changes in spatiotemporal patterns of benthic primary production but also to changes in biodiversity of life under water and ecosystem services including food supply. Satellite MPB data allows for adequate choices in selecting coastal biodiversity conservation and coastal food supply.
Highlights
Coastal wetlands are highly productive marine systems, supporting high rates of secondary production and providing food for higher trophic levels as well as supplying important services to humans [1, 2]
Primary production by benthic microalgae is especially important in tidal areas with a high proportion of mudflats that emerge during low tide [8,9,10] and in coastal areas that are characterised by turbid waters where production by pelagic microalgae is relatively low [11]
No significant relationship between chlorophyll-a concentrations and Normalized Difference Vegetation Index (NDVI) was found for the campaign in September 2018, in contrast to that found for April and July 2019 (Table 7)
Summary
Coastal wetlands are highly productive marine systems, supporting high rates of secondary production and providing food for higher trophic levels as well as supplying important services to humans [1, 2]. Worldwide, these systems are under intense pressure from coastal development, coastal erosion, dredging activities and reduced sediment fluxes from rivers [2, and references ]. Tidal flats harbour microphytobenthos (MPB), which fuels coastal food webs [5] These benthic microalgae are food for a variety of animals that graze on the algae directly from the sediment surface or filter them out of the water when resuspended by currents and wind-driven waves [6]. Primary production by benthic microalgae is especially important in tidal areas with a high proportion of mudflats that emerge during low tide [8,9,10] and in coastal areas that are characterised by turbid waters where production by pelagic microalgae is relatively low [11]
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